Imaging the choriocapillaris with SS-OCT at MHz A-scan rates

Abstract

Purpose : Resolving individual capillaries of the choriocapillaris with swept source optical coherence tomography (SS-OCT) angiography is challenging and has so far been limited to very small field of views (FOV). We present a MHz SS-OCT prototype, which provides high-contrast choriocapillaris (CC) images over FOV of up to 12mm x 12mm.

Methods : We developed a 1060nm SS-OCT prototype, capable of A-scan rates up to 1.7MHz. Its lateral resolution was 7.8μm and its axial resolution was 9 μm in tissue. We acquired Angio 12mm x 12mm scans with a sampling of 1024 (A-scans) x 1024 (B-scans) x 4 (B-scan repetitions) and Angio 2.2mm x 2.2mm scans with a sampling of 320x300x10. To create CC en face images, we segmented the RPE and integrated over 3 pixels, corresponding to a 20μm slab below the RPE fit. To compare our image quality to state of the art images, we acquired Angio 12 mm x 12 mm and Angio 3 mm x 3 mm scans of the same eye, using the 100 kHz mode of the Plex® Elite 9000 (ZEISS, Dublin, CA).

Results : In Fig. 1a&b Angio 12 mm x 12 mm scans of the CC acquired at 100 kHz (a) and 1.7 MHz (b) are presented side by side. Especially when comparing the magnified views in the blue boxes, one can appreciate significantly more contrast in the higher resolution 1.7MHz images. Fig. 1c-f show Angio scans over smaller FOV with higher sampling density. The Angio 3mm x 3mm scan with the commercial instrument is shown in c&d for the superficial and CC slab respectively. A corresponding Angio 3mm x 3mm image of the superficial slab is shown in Fig. 1f. Fig. 1e shows a 2.2mm x 2.2mm MHz Angio image of the CC at the location indicated by the yellow box in Fig. 1b. This illustrates that not only the sampling rate and lateral resolution, but also the sampling density strongly impact the ability to resolve individual capillaries of the CC.

Conclusions : We demonstrated that MHz A-scan rates paired with improved lateral resolution, can significantly improve the resolution of individual capillaries in the CC over a large FOV. We further show that increasing the lateral sampling and the number of B-scan repetitions can again significantly enhance the contrast of CC images.

This is a 2021 ARVO Annual Meeting abstract.

 

a) Angio 12mm x 12mm CC scan using commercial instrument, b) Angio 12mm x 12mm CC scan using MHz prototype, c) Angio 3mm x 3mm retina using commercial instrument, d) Angio CC image of the same scan as in c), e) densely sampled Angio CC using MHz prototype, f) Angio 3mm x 3mm retina using MHz prototype